Terephthalic acid (TA) and other aromatic carboxylic acids may be used in the manufacture of polyesters (e.g., via their reaction with ethylene glycol and/or higher alkylene glycols). Polyesters in turn may be used to make fibers, films, containers, bottles, other packaging materials, molded articles, and the like.
In commercial practice, aromatic carboxylic acids have been made by liquid phase oxidation of methyl-substituted benzene and naphthalene feedstocks in an aqueous acetic acid solvent. The positions of the methyl substituents correspond to the positions of carboxyl groups in the aromatic carboxylic acid product. Air or other sources of oxygen (e.g., typically in a gaseous state) have been used as oxidants in the presence, for example, of a bromine-promoted catalyst that contains cobalt and manganese. The oxidation is exothermic and yields aromatic carboxylic acid together with by-products, including partial or intermediate oxidation products of the aromatic feedstock, and acetic acid reaction products (e.g., methanol, methyl acetate, and methyl bromide). Water is also generated as a by-product.
Pure forms of aromatic carboxylic acids are oftentimes desirable for the manufacture of polyesters to be used in important applications (e.g., fibers and bottles). Impurities in the acids (e.g., by-products generated from oxidation of aromatic feedstocks and, more generally, various carbonyl-substituted aromatic species) are thought to cause and/or correlate with color formation in polyesters made therefrom, which in turn leads to off-color in polyester converted products. Aromatic carboxylic acids having reduced levels of impurities may be made by further oxidizing crude products from liquid phase oxidation as described above at one or more progressively lower temperatures and oxygen levels. In addition, partial oxidation products may be recovered during crystallization and converted into the desired acid product.
Pure forms of terephthalic acid and other aromatic carboxylic acids having reduced amounts of impurities—for example, purified terephthalic acid (PTA)—have been made by catalytically hydrogenating less pure forms of the acids or so-called medium purity products in solution at elevated temperature and pressure using a noble metal catalyst. In commercial practice, liquid phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid, and purification of the crude product, are oftentimes conducted in continuous integrated processes in which crude product from the liquid phase oxidation is used as a starting material for the purification.
In conventional purification units, crude aromatic carboxylic acid is typically mixed with water to form a purification reaction mixture prior to its introduction to the purification reactor. The mixing occurs in a feed mix vessel that is maintained at ambient pressure in order to allow rerun aromatic carboxylic from the vessel and to enable use of a screw conveyor feed of crude aromatic carboxylic from an intermediate silo to the feed mix vessel, which requires the feed mix vessel to be pressure-equilibrated with the silo which operates at ambient pressure. When the feed mix vessel is at ambient pressure, the highest possible temperature of the water in the vessel is about 100° C., the boiling point of water at ambient pressure. The purification reaction mixture must be pre-heated prior to its introduction into the purification reactor, which typically runs at 250° C. to 300° C. This heating required to raise the temperature of the purification reaction mixture from 100° C. or less to at least 250° C. adds to the variable cost of the integrated process for manufacturing purified aromatic carboxylic acids.
There continues to be a need to reduce the overall costs of manufacturing aromatic carboxylic acids.